Low rate telemetry channel

ABSTRACT

Low rate telemetry transmission between a transmission side and a receiving side is accomplished at the transmission side on digital information in a channel at a predetermined bit rate by selecting a predetermined frame thereof and outputting a selected one of at least two digital information elements desired for transmission with and in addition to those in the frame by means of a key generator which outputs a known key to the transmission and receiving sides and defined by a sequence of digital information that uniquely identifies the selected digital information elements by first processing the digital information element with the key in a first reversible mathematical manipulation to provide a first output and secondly performing a second reversible mathematical manipulation of the first output and the digital information in the frame to obtain an encoded digital information sequence which is then transmitted from the transmission side and received by the receiving side where it is decoded by reversing the mathematical manipulations for obtaining the frame of digital information in the channel and the key and form identifying the selected element from the key.

FIELD OF THE INVENTION

This invention relates to a method of low rate telemetry, and toapparatus therefor.

BACKGROUND TO THE INVENTION

Many digital transmission systems implement some kind of low ratetelemetry, using any of a number of methodologies. One approach which isused for such low rate telemetry is to increase the rate of informationtransmission by inserting telemetry information in the main data stream.This technique requires an increase in the rate of data transmissionacross the channel, and a corresponding increase in the totaltransmission bandwidth occupied by the system.

Another approach on systems such as optical fiber transmission systems,is to transmit telemetry information in the main data stream. Thistechnique also requires an increase in the rate of data transmissionacross the channel, and a consequent increase in the total transmissionbandwidth occupied by the system.

A further approach on optical fiber systems such as transmissionsystems, is to transmit telemetry information by an analog modulationscheme. The bandwidth occupied by this channel, must also be added tothe total bandwidth occupied by the transmission system, with aresulting increase in bandwidth.

A third approach relies on some form of pre-coding strategy, in whichcoding violations can be used to indicate the presence of a binary `1`or binary `0` on the telemetry channel. Although this technique does notrequire an increase in the transmission rate of the coded data, thebandwidth occupied by the coded data signal will be more than theminimum bandwidth required by the Nyquist criterion.

In this specification a `field` is a set of numbers, and is denoted byGF(n) , where n denotes the number of numbers in the set. Thus GF(2)will imply a set of 2 numbers, i.e. (0,1). Where n is an integer.

Also, a `closed operation` is said to be closed on some field, GF(n), ifthe input to the operation, as well as the output of the operation aremembers of GF(n).

DISCLOSURE OF INVENTION

It is an object of this invention to provide a method of low ratetelemetry and apparatus therefor.

In accordance with this invention there is provided low rate telemetrytransmission method comprising:

transmitting digital information on a channel at a pre-determined bitrate;

selecting a pre-determined frame of digital information in the channel;

selecting one of at least two digital information elements desired to betransmitted with and in addition to those in the frame;

obtaining a key known at transmission and receiving sides defined by asequence of digital information which after predetermined processingsubstantially uniquely identifies the selected digital informationelement;

performing as part of said predetermined processing a reversiblemathematical manipulation of the key and the digital information elementto produce a first output; performing a reversible mathematicalmanipulation of the first output and the digital information in theframe, to obtain an encoded digital information sequence, andtransmitting that encoded sequence; and,

receiving and decoding the transmitted encoded sequence by reversing themathematical manipulation to obtain the frame of digital information inthe channel and the key, and identifying the selected element from thekey.

Preferably the mathematical manipulation is a closed operation, and thedigital information in the frame is converted to a form which provides aclosed set for further mathematical manipulation.

Further preferably, the conversion is by taking the data as a vector ofthe length n members each of N elements, and forming a vector therefromof length N with n-tuple integer elements, and applying offset binarymapping to convert the data to a closed set.

There is provided for the reversible mathematical function to beselected to add redundancy to the said one digital information element.

Where this is done, and offset binary mapping is applied, themathematical manipulation to obtain the encoded digital informationsequence is a closed addition of the first output and the digitalinformation.

There is also provided for the reverse mathematical manipulation fordecoding the receive sequence includes the expected value of theperiodic crosscorrelation between the received sequence and the key.

Further, the reverse mathematical manipulation to decode the receivesequence includes a threshold determination of the expected value, andthe threshold determination of the expected value is the determinationof the polarity thereof, and the said one digital information element isone of two polarity alternatives.

Preferably the value of the key is chosen to have an autocorrelationvalue with peaks at a number of shift values, such that theautocorrelation of the key is much greater than the crosscorrelation ofthe data in closed set configuration, and the key itself.

The invention also provides apparatus for low rate telemetrytransmission between a transmission side and a receiving side,comprising:

means for transmitting digital information on a channel at apre-determined bit rate and for selecting a pre-determined frame ofdigital information in the channel;

means for outputting a selected digital information element of at leasttwo digital information elements desired to be transmitted with and inaddition to information elements in the frame;

a key generator for outputting a known .key to the transmission andreceiving sides, the key being defined by a sequence of digitalinformation which after predetermined processing substantially uniquelyidentifies the selected digital information element;

first processing means for performing as part of said predeterminedprocessing a first reversible mathematical manipulation of the key andthe digital information element to produce a first output;

second processing means for performing a second reversible mathematicalmanipulation of the first output and the digital information in theframe, to obtain an encoded digital information sequence, andtransmitting that encoded sequence; and,

receiving and decoding means for receiving and decoding the transmittedencoded- sequence by reversing the mathematical manipulation to obtainthe frame of digital information in the channel and the key, andidentifying the selected element from the key.

The second processing means may be arranged to perform a closedmathematical manipulation operation.

The second processing means is arranged to convert the digitalinformation in the frame to a closed set conversion by taking the dataas a vector of the length n members each of N elements, and forming avector therefrom of length N with n-tuple integer elements, and applyingoffset binary mapping to convert the data to closed set.

Preferably, the first processing means is arranged to provide thereversible mathematical function to add redundancy to the said onedigital information element.

In this instance, and where binary offset mapping is applied, themathematical manipulation to obtain the encoded digital informationsequence is a closed addition of the first output and the digitalinformation.

The receiving and decoding means may be arranged to decode the receivesequence to obtain the expected value of the periodic crosscorrelationbetween the received sequence and the key, and preferably is arranged toobtain a threshold determination of the expected value.

There is provided for the threshold determination of the expected valueto be the determination of the polarity thereof, and the said onedigital information element to be one of two polarity alternatives.

The key generator can select the key to have an autocorrelation valuewith peaks at a number of shift values, such that the autocorrelation ofthe key is much greater than the crosscorrelation of the data in aclosed set configuration, and the key itself.

The invention extends to a signal transmitted in accordance with themethod and separately, or additionally, the apparatus as set out above.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of a best mode embodiment thereof, as illustrated in theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

A preferred embodiment of the invention is described below by way ofexample only, and with reference to the accompanying sole FIGURE, whichis a functional block diagram of a telemetry system.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a data vector X_(k) is to be transmitted across achannel, and is input into a processing operation (10) to produce anoutput of X_(k). A known code vector Y is input to a multiplicationfunction (11) which has a further input of a low rate telemetryinformation bit W. The output of the multiplication process (11) isZ_(k), which is added to X_(k) in addition means (12), to produce anoutput V_(k). V_(k) is transmitted along channel (13), where it is takento a correlation function (14). The received signal V_(k) is correlatedwith the known vector Y to produce an output Σ_(n) in (15). The Σ_(n)output is tested by a polarity detector (16) for its polarity, and theoutput therefrom is the low rate telemetry bit W.

The output W as well as the known vector Y are multiplied atmultiplication means (17) to produce an output Z_(k) which is taken to asubtraction means (18). At the subtraction means, the received vectorV_(k) has Z_(k) subtracted from it, to produce X_(k). The X_(k) signalis taken to processing unit (20) which provides the opposite processingto that of unit (10) to have an output of X_(k).

The operation of this system is further described with reference toFIG. 1. The data that is to be transmitted across the channel at time kis represented by the vector X_(k) of length nN. Processing operation(10) then treats successive n-tuples of X_(k) as either offset-binary ortwo's complement numbers to form a vector X_(k) of length N with integerelements. For example, let

    X.sub.k =(0,1,1,0,1,1,0,0,0,1,1,0,1,0,0)                   equation (1)

with n=3 and N=5. The following n-tuples can then be extracted fromequation (1) to form

    X.sub.k =(011, 011, 000, 110, 100)                         equation (2)

and using an offset binary mapping, equation (2) becomes

    X.sub.k =(0,0-3,3,1)                                       equation (3)

In parallel with this operation, the vector Z_(k) is formed by encodingthe low rate telemetry information bit W with a known code vector Y oflength N. In general, the elements of Y will be binary, and in thiscase, the elements of Y will be ±1. In this embodiment the encoding isdone by multiplying each element of Y with W in multiplication process(11). This process is exactly the same as the spreading operation in adirect sequence spread spectrum system and adds redundancy to thetelemetry bit. The encoding operation used is not limited to such anoperation, and any encoding operation that adds redundancy to W can beused. For example, let

    Y.sub.k =(1,-1,1,-1,1)                                     equation (4)

and W be equal to -1. Then Z_(k) will be

    Z.sub.k =W×Y.sub.k                                   equation (5)

Following this, X_(k) and Z_(k) are added in addition means (12). Sincean offset-binary representation of the data is used, this closedaddition will yield the following

    V.sub.k =X.sub.k +Z.sub.k                                  equation (6)

    V.sub.k =(-1,1,4,4,0)

as a value of -4 will map to 4, and a value of 5 to -3 respectively. Theaddition can be regarded as normal linear addition, because the resultof the addition will mostly be representable by the n bits used torepresent X_(k). At this state, V_(k) has N elements, each representedby an n-tuple, yielding a total message length of nN bits, the samenumber as in the input sequence X_(k). Therefore the encoding operationhas not increased the required bit rate.

At the receiver, the expected value of the periodic crosscorrelationbetween V_(k) and Y is calculated and compared to a threshold todetermine the value of W. The correlation takes place at correlationfunction (15). Assuming that the addition of equation 7 is linear, thisprocess can be written as ##EQU1## where 1 denotes the shift variableand E ! denotes the expected value operation.

E C_(X).sbsb.k_(Y) ! will approximate zero as N increases asC_(X).sbsb.k_(Y) is a zero-mean Gaussian random variable. However, theperiodic autocorrelation of Y depends only on the choice of Y.Therefore, if Y is chosen to have an autocorrelation with peaks at anumber of shift values, E C_(YY) ! will be much greater than EC_(X).sbsb.k_(Y) !. This means that the sign of E T_(l) ! will dependsolely on W. If E T_(l) ! is smaller than zero, W will be equal to minusone. This threshold is detected by the polarity detector (16). Thetelemetry information is thus recovered at this position.

To recover the main data stream, the variable Z_(k) (see equation 6) isagain formed at the decoder by multiplying Y with W in multiplicationmeans. This is possible because W is now known. Subtracting Z_(k) fromV_(k) in subtraction will then yield ##EQU2##

After processing at operation (20) X_(k) is reversed back to X_(k) andthe original data stream is recovered.

Simulations of the above embodiment were performed for code lengths ofN=64 and N=128 to prove the feasibility of the system. The performanceof the system was also tested when used in conjunction with shortrepetition codes for error correction. In each case, a majority logicdecision was made as to the value of W. Table 1 shows the simulationresults for different code lengths (N) and different repetition codelengths. The bit error rate (BER) values given, are the BER of thetelemetry channel. No errors were introduced into the received signal.The BER of the data stream will be slightly better than that of thetelemetry channel, and therefore the BER of the telemetry channel can beseen as an upper bound for the BER of the data channel. Clearly the BERis a function of N, and in fact, by increasing N the BER can be made aslow as required.

                  TABLE 1                                                         ______________________________________                                        BER performance of telemetry system.                                                      Repetition Code                                                   N           Length      BER                                                   ______________________________________                                        64          1           5.3 × 10.sup.-2                                             3           8.9 × 10.sup.-3                                             5           1.4 × 10.sup.-3                                 128         1           1.2 × 10.sup.-2                                 128         3           3.9 × 10.sup.-4                                 128         5           1.3 × 10.sup.-5                                 ______________________________________                                    

The system exhibits no increase in the transmission bandwidth above theminimum Nyquist bandwidth. Information is added to the data stream usingoperations closed in GF (2^(n)), and thus, although the actual values ofthe data stream are changed, the nature of the data stream remainsunchanged since the data remains binary and the data rate has not beenaltered. The encoded telemetry information is added element by elementto the data stress without increasing the transmission rate of the data,and is included in every element of the data signal. This data isrecovered by examining every bit using a periodic correlation function.

Although the invention has been shown and described with respect to abest mode embodiment thereof, it should be understood by those skilledin the art that the foregoing and various other changes, omissions andadditions in the form and detail thereof may be made therein withoutdeparting from the spirit and scope of the invention.

I claim:
 1. A low rate telemetry transmission method for transmission ofdigital information between a transmission side and a receiving side,comprising:transmitting digital information on a channel at apredetermined bit rate; selecting a pre-determined frame of digitalinformation in the channel; selecting one of at least two digitalinformation elements desired to be transmitted with and in addition tothose in the frame; obtaining a key, known at transmission and receivingsides defined by a sequence of digital information which afterpredetermined processing substantially uniquely identifies the selecteddigital information element; performing as part of said predeterminedprocessing a first reversible mathematical manipulation of the key andthe digital information element to produce a first output; performing asecond reversible mathematical manipulation of the first output and thedigital information in the frame, to obtain an encoded digitalinformation sequence, and transmitting that encoded sequence; and,receiving and decoding the transmitted encoded sequence by reversing thefirst and second mathematical manipulations for obtaining the frame ofdigital information in the channel and the key, and for identifying theselected element from the key.
 2. A method as claimed in claim 1 inwhich the mathematical manipulation is a closed operation.
 3. A methodas claimed in claim 2 in which the digital information in the frame isconverted to a form which provides a closed set for further mathematicalmanipulation.
 4. A method as claimed in claim 3 in which the conversionis by taking the digital information as a vector of a length having nmembers each of N elements, and forming a vector therefrom of length Nwith n-tuple integer elements, and applying offset binary mapping toconvert the digital information to a closed set.
 5. A method as claimedin claim 4 in which the first reversible mathematical manipulation isselected to add redundancy to the said one digital information element.6. A method as claimed in claim 5 in which the second reversiblemathematical manipulation to obtain the encoded digital informationsequence is a closed addition of the first output and the digitalinformation.
 7. A method as claimed in claim 6 in which the reversing ofthe mathematical manipulation for receiving and decoding the transmittedencoded sequence includes an expected value of a periodiccrosscorrelation between the received sequence and the key.
 8. A methodas claimed in claim 7 in which the reversing of the mathematicalmanipulation for receiving and decoding the transmitted encoded sequenceincludes a threshold determination of the expected value.
 9. A method asclaimed in claim 8 in which the threshold determination of the expectedvalue is a determination of a polarity thereof, and the said selectedone of at least two digital information elements is one of two polarityalternatives.
 10. A method as claimed in claim 9 in which a value of thekey is chosen to have an autocorrelation value with peaks at a number ofshift values, such that an expected value of an autocorrelation of thekey is much greater than the crosscorrelation of the digital informationin a closed set configuration, and the key itself.
 11. Apparatus for lowrate telemetry transmission between a transmission side and a receivingside, comprising:means for transmitting digital information on a channelat a pre-determined bit rate and for selecting a pre-determined frame ofdigital information in the channel; means for outputting a selecteddigital information element of at least two digital information elementsdesired to be transmitted with and in addition to information elementsin the frame; a key generator for outputting a known key to thetransmission and receiving sides, the key being defined by a sequence ofdigital information which after predetermined processing substantiallyuniquely identifies the selected digital information element; firstprocessing means for performing as part of said predetermined processinga first reversible mathematical manipulation of the key and the digitalinformation element to produce a first output; second processing meansfor performing a second reversible mathematical manipulation of thefirst output and the digital information in the frame, to obtain anencoded digital information sequence, and transmitting that encodedsequence; and, receiving and decoding means for receiving and decodingthe transmitted encoded sequence by reversing the first and secondmathematical manipulation for obtaining the frame of digital informationin the channel and the key, and for identifying the selected elementfrom the key.
 12. Apparatus as claimed in claim 11 in which the secondprocessing means is arranged to perform a closed mathematicalmanipulation operation.
 13. Apparatus as claimed in claim 12 in whichthe second processing means is arranged to convert the digitalinformation in the frame to a closed set conversion by taking the dataas a vector of a length having n members each of N elements, and forminga vector therefrom of length N with n-tuple integer elements, andapplying offset binary mapping to convert the digital information to aclosed set.
 14. Apparatus as claimed in claim 13 in which the firstprocessing means is arranged to provide the reversible mathematicalfunction to add redundancy to the said one digital information element.15. Apparatus as claimed in claim 14 in which the second mathematicalmanipulation to obtain the encoded digital information sequence is aclosed addition of the first output and the digital information. 16.Apparatus as claimed in claim 15 in which the receiving and decodingmeans is arranged to decode the transmitted encoded sequence to obtainan expected value of a periodic crosscorrelation between the receivedsequence and the key.
 17. Apparatus as claimed in claim 16 in which thereceiving and decoding means is arranged to obtain a thresholddetermination of the expected value.
 18. Apparatus as claimed in claim17 in which the threshold determination of the expected value is adetermination of a polarity thereof, and the said determination of saidpolarity of said selected digital information element is one of twopolarity alternatives.
 19. Apparatus as claimed in claim 18 in which thekey generator can select the key to have an autocorrelation value withpeaks at a number of shift values, such that an expected value of anautocorrelation of the key is much greater than the crosscorrelation ofthe digital information in a closed set configuration, and the keyitself.